Expand.
We expand the parentheses using the distributive property : $ A(B+C+D)= A\cdot B+ A\cdot C+ A\cdot D$ We can also think about the problem using an area model: $x^2$ $-5x$ $6$ $3x$ Here's how the solution goes, algebraically: $\begin{aligned} &\phantom{=}{3x}(x^2-5x+6) \\\\ &={3x}(x^2)+{3x}(-5x)+{3x}(6) \\\\ &=3x^3-15x^2+18x \end{aligned}$ Here's how the solution looks in terms of the area model: $3x^3$ $-15x^2$ $18x$ $x^2$ $-5x$ $6$ $3x$ In conclusion, $ 3x(x^2-5x+6)=3x^3-15x^2+18x$